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Abstract

The photo-Dember effect is a source of impulsive THz emission following femtosecond pulsed optical excitation. This emission results from the ultrafast spatial separation of electron-hole pairs in strong carrier gradients due to their different diffusion coefficients. The associated time dependent polarization is oriented perpendicular to the excited surface which is inaptly for efficient out coupling of THz radiation. We propose a scheme for generating strong carrier gradients parallel to the excited surface. The resulting photo-Dember currents are oriented in the same direction and emit THz radiation into the favorable direction perpendicular to the surface. This effect is demonstrated for GaAs and In0.53Ga0.47As. Surprisingly the photo-Dember THz emitters provide higher bandwidth than photoconductive emitters. Multiplexing of phase coherent photo-Dember currents by periodically tailoring the photoexcited spatial carrier distribution gives rise to a strongly enhanced THz emission, which reaches electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.

Figures (5)

(a), (b) Principle of the THz emission from lateral photo-Dember currents. In the case of a bare semiconductor surface the gradient of the photoexcited electrons (blue spheres) and holes (red spheres) and the resulting photo-Dember polarization PDember is directed perpendicular to the surface (a). In the case of a partially covered semiconductor surface a strong carrier gradient is achieved at the edge of a metalized stripe, resulting in a photo-Dember polarization parallel to the surface (b). In both pictures the green lobes indicate the dipole radiation patterns of the arising THz radiation. (c) Sketch of the sample with two gold stripes on GaAs used for the line scans shown in the inset of Fig. 2. The green cones show the forward direction of the resulting THz radiation. (d) Sketch of a multiplexed photo-Dember emitter as used for the data shown in Fig. 5.

THz transients emitted from the left (black) and right (red) metal-semiconductor edge. Left and right are defined as in the schematic drawing shown in Fig. 1(c). The transients are normalized to the maximum electric field value at the right edge. The inset shows the peak-to-peak amplitude of the emitted THz electric field when the metal-semiconductor edge is translated underneath the laser spot. The line scans are normalized to the peak-to-peak amplitude at 12.39 mm.

Excitation density dependence of the THz emission of the same sample as used in Fig. 2. The data are normalized to the peak-to-peak amplitude at the highest excitation density. The different colors refer to different spot sizes as given in the text. The inset shows the excitation density dependence of a large area photoconductive emitter with interdigitated electrodes. Again the data are normalized to the value at the highest excitation density. The descriptions of the axis are the same as in the main plot.

Comparison of a large area photoconductive emitter with interdigitated electrodes with a multiplexed photo-Dember emitter. The micro-structure of the multiplexed photo-Dember emitter is shown in Fig. 1(d). Figure (a) shows the normalized THz transients, (b) the normalized Fourier amplitude spectra. The inset shows the same data not normalized for absolute comparison on a log scale.